Prune 'left' members of rooms in CS API from initial /sync? (SYN-487)

[matrixbot]

Clients surely never care about users who are in state 'left' in a room. Can we not just prune them from the state that's passed to the client?

(Imported from https://matrix.org/jira/browse/SYN-487)

(Reported by @ara4n)

[matrixbot]

Jira watchers: @ara4n

[ara4n]

@erikjohnston can you remind me why we're not doing this? Is it because /messages may get confused if you try to back-paginate from an initial /sync, and members suddenly pop into life who weren't even mentioned in the initial /sync response?

[babolivier]

Fixed by #2970